Low Equivalent weight ionomer

ABSTRACT

An ionomer and a process for forming the ionomer such that the ionomer has (1) low equivalent weight; below 950, preferably between 625 and 850, and most preferably between about 700 and about 800; and (2) high conductivity, (greater than 0.15 S/cm). In an alternative embodiment, the ionomer has (1) low equivalent weight; below 950, preferably between 625 and 850, and most preferably between about 700 and about 800; and (2) acceptably low hydration, (less than about 75 weight percent). These ionomers are adapted to be processed into thin films that have acceptable physical stability. They are thus extremely well-suited for low humidity or high temperature fuel cell applications.

RELATED APPLICATION

The present application is a divisional application of U.S. patentapplication Ser. No. 10/011,242, filed Dec. 5, 2001.

FIELD OF THE INVENTION

The present application relates to fluorinated ionomers, andspecifically to fluorinated ionomers of low equivalent weight that haverelatively low hydration and can be processed into thin films. Thefluorinated ionomers are preferably perfluorinated compounds suitablefor applications where reasonably high ionic conductivity in lowhumidity environments is desirable. One such application is in solidpolymer electrolyte fuel cells.

BACKGROUND OF THE INVENTION

Solid polymer ionic membranes or films have been well known in the artfor many years. These polymers are typically characterized by high ionicconductivity, i.e., the rapid transport of ionic species, e.g., protons,at relatively modest temperatures, e.g., 50-90 degrees C. Additionally,it is desirable for such ionically conducting polymers to be made in theform of membranes or thin films. In so doing, the resistance to ionictransport, which is a function of the film thickness, can be reduced.Fluoropolymer compositions are particularly desirable for such uses, andare disclosed, for example, in U.S. Pat. No. 3,282,875, U.S. Pat. No.4,358,545 and U.S. Pat. No. 4,940,525.

The instant invention relates to ionomers, which as used herein means aperfluorinated polymer containing acid groups or acid derivatives easilyconverted to acid groups such that the acid form of the polymer inmembrane form has a room temperature ionic conductivity greater than1×10⁻⁶ S/cm. As used herein the acid form of an ionomer means thatsubstantially all the ion exchange groups, e.g., SO₃ ⁻ or sulfonicgroups, are protonated. One important parameter used to characterizeionomers is the equivalent weight. Within this application, theequivalent weight (EW) is defined to be the weight of the polymer inacid form required to neutralize one equivalent of NaOH. As is known inthe art, one can also convert the equivalent weight into otherparameters that may be useful. For example the ion exchange capacity,which is 1000 divided by the equivalent weight; or the mole fraction ormole percent of ionomer in a copolymer of ionomer and non-ionomer.Higher EW means that there are fewer active ionic species (e.g.,protons) present. If it takes more of the polymer to neutralize oneequivalent of hydroxyl ions there must be fewer active ionic specieswithin the polymer. Because the ionic conductivity is generallyproportional to the number of active ionic species in the polymer, onewould therefore like to lower the EW in order to increase conductivity.

Lowering the equivalent weight has previously not been a practicalapproach to making useful membranes. This is because with fluoropolymerscurrently known, as the equivalent weight goes down, the amount of water(or solvent) that the polymer absorbs goes up. The amount of waterabsorbed by the polymer is called the degree of hydration or hydration.It is expressed as the weight percent of water absorbed by the polymerunder a given set of conditions, for example, after immersion in roomtemperature water for two hours. A higher degree of hydration isdesirable up to a point because it tends to increase the ionicconductivity of the membrane. Correspondingly, lowering the degree ofhydration has traditionally meant decreasing the conductivity. But thereis a limit to the amount of water or solvent such fluoropolymermembranes can contain. If too much water is present, the film may losemuch of its physical integrity, becoming gel-like with little or norigidity. In the extreme, the polymer may completely disintegrate. Inaddition, depending on the exact polymer composition, low EWfluoropolymer ionomers may even partially or completely dissolve inwater. Furthermore, even if the films were to be stable, too high ahydration would tend to dilute the number of ions present forconduction, thereby lowering the conductivity. Thus, there is an optimaldegree of hydration that is high enough to provide the highest possibleconductivity, while not so high that the films become physicallyunstable when hydrated.

Thus, one would like to decrease the equivalent weight of thesefluoropolymers to increase their conductivity, but heretofore could notpractically do so because the degree of hydration and/or watersolubility was too high to form practical membranes.

Various approaches have been used to circumvent this limitation. In U.S.Pat. No. 5,654,109, U.S. Pat. No. 5,246,792, U.S. Pat. No. 5,981,097,U.S. Pat. No. 6,156,451, and U.S. Pat. No. 5,082,472 various forms oflayered composite membranes are suggested. In '109, the use of a bilayeror trilayer composite ion exchange membranes is suggested where theouter layer or layers are lower equivalent weight for improvedelectrical performance, while the core layer has a higher EW thatprovides strength. A similar approach is suggested in '792 but the filmsare layers are characterized by their glass transition temperaturesinstead of EW. Three or more layers with variable ion exchange ratio (aparameter proportional to EW) is proposed in '097. In '472 a process toform a membrane is taught whereby a perfluorinated ionomer is laminatedto a porous expanded PTFE membrane, followed by impregnation of a lowequivalent weight ionomer (e.g., 920-950 EW) into that laminate. Becausethe impregnation is performed with a solution with low solids content(e.g., 2%), the amount of low equivalent weight material in the finalproduct is relatively low. Although each of these approaches may offersome improvement over a monolithic single layer fluoropolymer membrane,they all involve the use of rather complex, composite, multilayerstructures that can be difficult and/or expensive to process.

Approaches to modifying the fluoropolymer itself have also been taught,for example in U.S. Pat. No. 4,358,545 to Ezzell. The properties ofthese polymers are described in Moore and Martin, “Morphology andChemical Properties of the Dow Perfluorosulfonate Ionomers”,Macromolecules, vol. 22, pp. 3594-3599 (1989), and Moore and Martin,“Chemical and Morphological Properties of Solution-CastPerfluorosulfonate Ionomers”, Macromolecules, vol. 21, pp. 1334-1339(1988). The approach described in these references is to produceionomers with shorter side chains along the polymer backbone. Thisapproach is particularly desirable for use in coating processes (forexample, as described in U.S. Pat. No. 4,661,411 and U.S. Pat. No.5,718,947), but still suffers limitations for use as fluoropolymerionomer membranes. In particular, these polymers can still be difficultto form into acceptably thin, strong membranes from solution.

Another approach as described by various authors is to form co-polymersof tetrafluoroethylene and ionomers using variations of the well-knownemulsion polymerization (for example, the process disclosed in U.S. Pat.No. 3,282,875). In U.S. Pat. No. 5,608,022 to Nakayama et. al. and WO00/52060 to Bekarian, et. al., processes are taught to formfunctionalized, fluorinated co-polymers by dispersing fine droplets of afluorinated co-monomer before polymerizing with a traditional fluorinecontaining monomer, e.g. tetrafluoroethylene. In these processes, theformation of fine droplets of the co-monomer is a key to a successfulpreparation of the polymer. In WO 94/03503 to Barnes, et. al. the rateof addition of the tetrafluoroethylene monomer to the ionomer emulsionis controlled by either altering the concentration of the emulsionduring polymerization, varying the pressure of the tetrafluoroethylenegas during reaction, or varying the agitation of the reaction mixture.Barnes teaches that these approaches result in a product with higherutilization of the ionomer as determined by the property of equivalentweight distribution, which he defines as a ratio of EW determined bymeans of titration to that determined by nuclear magnetic resonance.Barnes et. al. Claims that this higher utilization leads to a higherRelative Hydration Product and higher Specific Conductivity. Both theseparameters were evaluated in the presence of 2.5 Molar sulfuric acid(2.5 M H₂SO₄), and therefore are not relevant to the current applicationwhere only hydrated polymer (in the absence of acid electrolyte) isconsidered.

In yet another approach taught in PCT WO 00/79629 an ionomeric polymeris intimately mixed with a structural film-forming polymer, such as aterpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidenefluoride (e.g., THV series available from Dyneon Corp., Oakdale, Minn.).It then is possible to form acceptably thin films using low equivalentweight ionomers. But, the degree of hydration is still relatively high,80-110%, when 800 EW starting ionomer is used (e.g., Table 1 in WO00/79629). Thus, these films might be expected to be relatively weakbecause of the high hydration.

Finally there is also a large body of art that describes approaches toforming non-ionomeric fluoropolymers. For the most part, this art is notrelevant to the instant invention described here because the productsproduced do not have substantial ionic conductivity, i.e., the ionicconductivity of these products is less than about 1×10⁻⁶ S/cm at roomtemperature.

Fluoropolymer ionically conducting membranes have been utilized in manydifferent applications. One application that has been widely suggestedis as electrolytic cell membranes for the electrolysis of sodiumchloride as disclosed, for example, in U.S. Pat. Nos. 4,358,545,4,417,969, and 4,478,695. Additionally, this generic class of polymersdescribed as fluoropolymer ionomers have been proposed for use ascoatings as described above in U.S. Pat. No. 4,661,411 to Martin et.al.; as wire insulation (e.g., in WO 90/15828); as replacements for acidcatalysts, primarily in organic synthesis as described in“Perfluorinated Resin sulfonic Acid (Nafion-H(R)) Catalysis inSynthesis”, by Olah, G. A., Iyer P. S. and Surya P. G. K., in Journal:Synthesis (Stuttgart), 1986 (7) 513-531, and in “Perfluorinated Resinsulfonic acid (Nafion-H) Catalysis in Organic Synthesis” by Yamato, T.,in Yuki Gosei Kagaku Kyokaishi/Journal of Synthetic Organic Chemistry,volume 53, number 6, June 1995, p 487-499; as a membrane for waterelectrolysis as described in Yen, R. S., McBreen, J., Kissel, G.,Kulesa, F. and Srinivasan, S. in the Journal of AppliedElectrochemistry, volume 10, pg. 741, 1980; as a membrane forelectrowinning as described, for example, in “The Use of Nation-415Membrane in Copper Electrowinning from Chloride Solution” by Raudsepp,R., and Vreugde, M., in CIM Bulletin, 1982, V75, N842, P122; in metalion recovery systems as described in product literature of Nafion®perfluorinated membrane case histories, DuPont Company, Polymer ProductsDepartment, Wilmington, Del. 19898; as a tube to continuously and veryselectively dry wet gas streams (see product literature from Perma Pure,Inc., Toms River, N.J.); and as components in polymer electrolytemembrane (PEM) fuel cells. In the latter case, they can function both asthe electrolyte or a component thereof, for example as described in byBahar et. al. in U.S. Pat. Nos. 5,547,551 and 5,599,614; and/or as acomponent in one or both of the electrodes of the MEA.

When the ion conducting polymers, or ionomers, are used as theelectrolyte in PEM fuel cells they conduct protons from one electrode tothe other. A common problem associated with such fuel cells is thatcontaminants such as carbon monoxide tend to poison the catalysts usedin the MEA. These contaminants can interfere with the flow of ionsbetween the electrodes and thus degrade the performance of the fuelcell.

One way to reduce the effect of carbon monoxide is to operate the fuelcell at an elevated temperature. This reduces the formation and/orincreases the destruction rate of potential contaminants and therebyallows more efficient electrode performance.

The problem with running at high temperatures, however, is that itvaporizes liquid water within the fuel cell, and in so doing, tends toreduce the degree of hydration in the membrane. As described above,decreasing the hydration lowers the ionic conductivity, thereby reducingthe efficiency of ion transport through the membrane and adverselyaffecting fuel cell operation. In fact, at lower temperatures, in PEMfuel cells using conventional ionomers the incoming gas streams areusually well-humidified in order to maintain a relatively high degree ofhydration. Only by adding the additional water in the form of humidityin the gases can the hydration be kept high enough to allow efficientfuel cell operation for long periods of time. However, as thetemperature gets close to, or above, the boiling point of water thisapproach becomes difficult and inefficient. Thus, an ionomer withrelatively low hydration and acceptably high ionic conductivity wouldrequire less ambient water to function as the electrolyte in PEM fuelcells. It could function efficiently both in lower humidity environmentsat lower temperatures, as well as at temperatures closer to and evenpotentially above the boiling point of water.

As described above, the known low equivalent weight ionomers have arelatively high hydration. They are also known to be partially orcompletely soluble in water as well. These factors would counsel againsttheir use in environments where water is produced, e.g. hydrogen-oxygenfuel cells, because these polymers tend to become physically unstable inthese environments. In addition, as described above and shown recently(WO 00/52060, Table 1) the ionic conductivity decreases as theequivalent weight goes down concomitant with a large increase inhydration. The ionic conductivity reported in WO '060 decreases by morethan 30% when the equivalent weight of the subject ionomer is reducedfrom 834 to 785.

Against this background of conventional wisdom, applicants havediscovered a low equivalent weight ionomer that has a combination ofvery high ionic conductivity while maintaining a relatively lowhydration. As a result, this invention makes possible the more effectiveuse of solid fluoropolymer membranes in existing applications such asthose described above. Additionally, new applications heretofore notpractical may become possible with this new, unique set ofcharacteristics. The instant invention is particularly valuable as anelectrolyte or component thereof, or as a component in the electrode ofpolymer electrolyte membrane fuel cells operating at high temperaturesor low humidities.

SUMMARY OF THE INVENTION

The present invention is a fluorinated ionomeric co-polymer comprising

-   -   (a) a substantially fluorinated backbone;    -   (b) pendant groups derived from an ionomeric monomer of the        formula    -    where X is F, Cl or Br or mixtures thereof; n is an integer        equal to zero, one or two; R_(f) and R_(f′) are independently        selected from the group of F, Cl, perfluoroalkyl radical, and        chloroperfluoroalkyl radical; Y is an acid group or a functional        group convertible to an acid group; a is zero or an integer        greater than zero; and b is an integer greater than zero; and    -   (c) pendant groups derived from a vinyl ether monomer that has        at least two vinyl ether groups of the form, CA₂═CB—O—, where        the vinyl groups are separated by greater than four atoms; A is        independently selected from the group containing F, Cl, and H;        and B is independently selected from F, Cl, H and OR_(i), where        R_(i) is a branched or straight chain alkane that may be        partially, substantially or completely fluorinated or        chlorinated;    -   wherein the concentration of the pendant groups derived from the        ionomeric monomer and the vinyl ether monomer is between about        10 mole percent and about 45 mole percent of the final product,        and wherein the concentration of the pendant groups derived from        the vinyl ether monomer is present in the final product between        about 0.05 and about 2 mole percent.

These ionomers are adapted to be processed into thin films that haveacceptable physical stability. They are thus extremely well-suited forlow humidity or high temperature fuel cell applications.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic of a room temperature ionic conductivity fixture.

FIG. 2 is a schematic of a high temperature conductivity cell.

FIG. 3 shows the results of a dynamic mechanical analysis study of anexemplary embodiment of the inventive ionomers showing the complexviscosity as a function of shear rate.

FIG. 4 is a cross-sectional view of a membrane formed from an ionomeraccording to an exemplary embodiment of the present invention.

FIG. 5 is a cross-sectional view of a composite membrane formed using anionomer according to an exemplary embodiment of the present invention.

FIG. 6 is a cross-sectional view of a membrane electrode assembly formedusing an ionomer according to an exemplary embodiment of the presentinvention.

FIG. 7 is a cross-sectional view of a fuel cell including an ionomeraccording to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In this invention, the addition of low levels of branch forming agentsduring a mini-emulsion polymerization process is described for producinga co-polymer of two or more monomers. The co-polymer comprises asubstantially fluorinated backbone and one or more ionomers, such thatthe product has unusually high ionic conductivity and relatively lowhydration. The branch forming agents are added at a level below thatrequired to produce an insoluble gel, yet have surprising andsubstantial effects on the final product.

The process used herein is generally known in the art as mini-emulsionpolymerization, as described for example in Chapter 20, MiniemulsionPolymerization by E. David Sudol and Mohamed S. El-Aasser, in EmulsionPolymerization and Emulsion Polymers, P. A. Lowell and M. S. El-Aasser,eds, John Wiley and Sons, Ltd, New York, 1997. In these processes, amini-emulsion, which is defined herein by a droplet size in an emulsionbetween 50 and 500 nm, is formed by subjecting an oil (herein anionomer)/water/surfactant/co-surfactant system to high shear mixing suchas that produced by an ultrasonifier, a Manton Gaulin homogenizer, or amicrofluidizer. This mini-emulsion is then subjected to a polymerizationreaction. The processes and products produced from these processes inthe present invention are substantially different than prior art, e.g.,that described in U.S. Pat. No. 5,608,022 to Nakayama et. al., and WO00/52060 to Bekarian, et. al, because of the presence of theco-surfactant. The use of a co-surfactant to aid in achieving amini-emulsion offers the advantage of using lower quantities of thesurfactant, which can be advantageous because high concentrations of thesurfactant may have detrimental effects on the final product.

Although the use of a co-surfactant in microemulsion polymerization offluoropolymers has been recognized in the art (e.g., see Wu et. al. inU.S. Pat. No. 6,046,271) the use of the a mini-emulsion process with aco-surfactant for the preparation of ionomeric polymers described hereinis novel. For example, in '271 Wu discloses a polymerization procedureof forming a microemulsion of at least one liquid perfluorinatedhydrocarbon compound; adding at least one gaseous free-radicalpolymerizable polymer to the microemulsion; and initiatingpolymerization by adding a free radical initiator to the mixture. Wu didnot anticipate the use of monomers with acid end groups describedherein, specifically disclosing only straight chain fluoro orchlorofluoro alkenes or vinyl ethers (e.g., see column 4, lines 20-28)that will give products that are not ionomers. Further, the addition ofbranch forming agents as used herein was also not anticipated. Theparticularly surprising result we have discovered is that by using anionomeric monomer in a mini-emulsion polymerization in the presence of aco-surfactant (instead of the micro-emulsion of straight chain alkenesdisclosed previously) together with very low levels of a branch formingagent, we are able to produce polymers that have unusually high ionicconductivity. This totally unexpected result, coupled with equallysurprising relatively low hydration of the resulting polymer, allow theproduction of ion conducting membranes of great value.

In a co-pending application to Wu et. al, entitled, “Low EquivalentWeight Ionomers”, an aqueous miniemulsion polymerization procedure isdescribed for producing a co-polymer with two or more monomerscomprising a substantially fluorinated backbone and one or moreionomers, such that the product has unusually high ionic conductivity.That application did not anticipate that it would be possible to usebranch forming agents at low levels to even further improve theproperties of the resulting polymer. The particularly surprising resultwe have discovered is that by adding a branch forming agent at a levelbelow that required to product branch forming during polymerization of aco-polymer with a substantially fluorinated backbone and one or moreionomers, we are able to produce polymers that have exceptionalproperties. In particular, the polymers can be easily formed into filmsthat have a combination of very high room temperature ionicconductivity, relatively low hydration, and acceptable physicalstability. This totally unexpected result has produced membranes ofgreat value.

In one embodiment of the instant invention, a perfluorinated hydrocarbonis used as a co-surfactant in a mini-emulsion polymerization processdescribed more fully below. A branch forming agent is introduced intothe polymerization reaction at low levels, which surprisingly gives riseto a set of very desirable properties in the resulting ionomer. Thepolymer particles so produced can easily be formed into thin films thathave unusually high ionic conductivity, greater than about 0.15 S/cm atroom temperature. Additionally, these films have relatively lowhydration, and relatively high physical stability when compared tosimilar films prepared by prior art.

The co-surfactants can be chosen from among those co-surfactants knownin the art, such as alcohols, amines or other amphiphilic molecules, orsalts. Single or multiple co-surfactants can be employed to facilitateformation of the mini-emulsion. A particularly preferable co-surfactantis one drawn from the class of perfluorinated hydrocarbons of lowmolecular weight that is liquid at the temperature at whichpolymerization is carried out. The molecular weight is preferably lessthan 2000. The perfluorinated hydrocarbon preferably has a boiling pointless than 300 degrees C. The perfluorinated hydrocarbon can be aperfluorinated saturated aliphatic compound such as a perfluorinatedalkane. It can also be a perfluorinated aromatic compound such asperfluorinated benzene; a perfluorinated alkyl amine such as aperfluorinated trialkyl amine; a perfluorinated cyclic aliphatic, suchas decalin or perfluoro tetradecahydrophenanthrene; or a heterocyclicaliphatic compound containing oxygen or sulfur in the ring, such asperfluoro-2-butyl tetrahydrofuran. Examples of perfluorinatedhydrocarbons include perfluoro-2 -butyltetrahydrofuran,perfluorodecalin, perfluoromethyldecalin, perfluorodimethyldecalin,perfluoromethylcyclohexane, perfluoro(1,3-dimethylcyclohexane),perfluorodimethyldecahydronaphthalene, perfluorofluoorene,perfluorotetracosane, perfluorokerosenes, octafluoronaphthalene,oligomers of poly(chlorotrifluoroethylene), perfluoro(trialkylamine)such as perfluoro(tripropylamine), perfluoro(tributylamine), orperfluoro(tripentylamine), and octafluorotoluene, hexafluorobenzene,perfluoro ethers or perfluorinated polyethers, and commercialfluorinated solvents, such as Fluorinert FC-77 or FC-75 produced by 3M.The fluorinated alkanes can be linear or branched, with a carbon atomnumber between 3 and 20. Oxygen, nitrogen or sulfur atoms can also bepresent in the molecules.

The fluorinated surfactant has the structure R_(g)EX, where R_(g) is afluorinated alkyl or a fluorinated polyether group with a carbon numberbetween 4 and 16, E is an alkylene group with a carbon number between 0and 4, and X is an anionic salt such as COOM, SO₃M, SO₄M, a cationicmoiety such as quarternary ammonium salt, or an arnphoteric moiety suchas aminoxide, or a non-ionic moiety such as (CH₂CH₂O)_(m)H; and M is H,Li, Na, K, or NH₄; and

-   m is a cardinal number of 2 to 40. One preferred fluorinated    surfactant is ammonium perfluoro octanoate.

The substantially fluorinated backbone of this invention can be apolymer prepared from a number of different monomers or co-monomers thathave a high fluorine concentration. These can include, but are notlimited to tetrafluoroethylene, and mixtures of tetrafluorethylene withone or more monomers selected from the group hexafluoropropylene,vinyledene fluoride, chlorotrifluoroethylene, perfluoropropylvinylether, perfluoromethylvinyl ether and ethylene. One preferred monomerused to form the substantially fluorinated backbone istetrafluoroethylene.

The ionomeric monomers used in the polymerization reaction aresubstantially fluorinated organic compounds containing at least onemoiety that has ionic functionality and at least one polymerizablegroup. Alternatively, the molecule may carry precursors that can beconverted into ionic functionality after the polymerization process iscomplete. Examples of monomers suitable for forming these ionomersinclude compounds having the formula

and the like which after polymerization form pendant groups on thesubstantially fluorinated backbone of the form

In the above, X is F, Cl or Br or mixtures thereof; n is an integerequal to one or two; R_(f) and R_(f′) are independently selected fromthe group of F, Cl, perfluoroalkyl radical, and chloroperfluoroalkylradical; Y is an acid group or a functional group convertible to an acidgroup; a is zero or an integer greater than zero; and b is an integergreater than zero. Examples of Y that include acid groups include, butare not limited to, sulfonic acid or its salt form, —SO₃Z; sulfonamide,—SO₂N(R₁)—; sulfonimide, —SO₂N(R₁)SO₂R₂; carboxylic acid, —CO₂Z;phosphonic acid, —PO₃H₂; and the like, wherein Z is H, or anycombination of cations including, but not limited to, ammonium ion,metallic ions; or organoammonium ions; R₁ is H, an alkyl group withcarbon number from 1 to 10, or a partially fluorinated alkyl group witha carbon number of 2-10; and R₂ is a perfluorinated alkyl chain withcarbon number from 1 to 8, which can optionally contain oxygen or otheratoms or groups that are stable to free radicals; or a perfluoroalkylgroup, which can also optionally contain oxygen or other atoms or groupsthat are stable to free radicals and is terminated with Y as it isdefined above. Examples of Y that are function groups convertible to anacid group include, but are not limited to, sulfonyl halide, —SO₂W;ester, —COOR; and the like, wherein W is F, Cl, or Br, and R is an alkylgroup with carbon number from 1 to 20. One preferred ionomeric monomeris CF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂—SO₂F that forms pendant groups havingthe formula, —O—CF₂CF(CF₃)—O—CF₂CF₂—SO₂F. For this particular ionomerwith TFE as the comonomer, the conversion between equivalent weight andmole percent of ionomer is given approximately byMole % ionomer=100/(n+1)where n, the number of backbone units per ionomer unit, is given byn=(equivalent weight of polymer−446)/100More generally for other functional monomers and other comonomers, n isgiven by $n = \frac{\begin{matrix}{\left( {{equivalent}\quad{weight}\quad{of}\quad{polymer}} \right) -} \\\left( {{molecular}\quad{weight}\quad{of}\quad{ionomeric}\quad{monomer}} \right)\end{matrix}}{\left( {{molecular}\quad{weight}\quad{of}\quad{comonomer}} \right)}$

The branch forming agent that is added in low levels comprises a monomerselected from a group of vinyl ether compounds. In this application, lowlevels of such agents are defined as levels that do not causesubstantial gellation or network formation of the resulting product. Thebranch forming agents include, but are not limited to, monomers thathave at least two vinyl ether groups of the form, CA₂═CB—O—, such thatthe vinyl groups are separated by greater than four atoms. Here, A isindependently selected from the group containing F, Cl, H; and B isindependently selected from F, Cl, H and OR_(i), where R_(i) is abranched or straight chain alkane that may be partially, substantiallyor completely fluorinated or chlorinated. Particularly preferable branchforming agents in this class are perfluorinated vinyl ether compounds,CF₂═CF—O—R_(h)—O—CF═CF₂, wherein the R_(h) group is a perfluorinatedalkane with carbon number ranging from 3 to 15. The R_(h) alkane carbonscan optionally be branched and/or may be inserted with some etherlinkages, such as —CF₂—O—CF₂—, or sulfur linkage, such as a sulfonimide,or other linkages that do not take part in polymerization. Theperfluorinated vinyl ether compounds can be produced fromhexafluoropropylene oxide and a perfluorinated di-acid fluoride bymethods known in the art.

Without being bound by any particular theory, it is believed that thepresence of the vinyl ether compound at low levels introduces long chainbranching into the resulting polymer. By introducing it at low levels,the formation of cross-linking is minimized. High levels ofcross-linking may be undesirable because it would be expected to producta polymer that will not easily be formed into the desirable membraneform of the polymer. At low levels, the concentration of the agent isnot sufficient to obtain much, if any, cross linking. Instead, the agentacts to form long chain branching of the ionomeric co-monomer, which inturn gives rise to the surprising and unexpected improvement inconductivity and degree of hydration.

The preparation of the branch forming agents used herein are well knownin the art, as disclosed for example in U.S. Pat. No. 3,291,843, whichis included herein by reference in its entirety. Example XVIII in '843illustrates one procedure for preparing one branch forming agentdisclosed herein, i.e., F₂C═CFO(CF₂)₅OCF═CF₂. Another process for theformation of a different branch forming agent is to start with2,2-difluoromalonic acid fluoride (O═CF—CF₂—CF═O) and hexafluoropropylene oxide. 2,2-difluoromalonic acid fluoride is prepared by directfluorination of malonic acid by F₂ gas. The addition reaction of onemolecule of 2,2-difluoromalonic acid fluoride and two molecules ofhexafluoro propylene oxide will produce one molecule ofO═CF—CF(CF₃)—O—CF₂CF₂CF₂—O—CF(CF₃)CF═O,which after a standard decarboxylation reaction (see for example,Example V in '843 or column 9, lines 24-38 in U.S. Pat. No. 5,463,005),becomes the desired branch forming monomer. If desirable, the productcan be reacted to a more stable form for long-term storage, for examplea brominated form where the vinyl groups are saturated with bromine,becoming for example in this case, BrCF₂CFBrOCF₂CF₂CF₂OCFBrCF₂Br. Thevinyl form can then be regenerated prior to use with standard appoaches,for example by flowing the brominated form over zinc metal.

The amount of branch forming agent added during the polymerization ofthe inventive product is low by usual practices in the art, being addedat levels so that the amount in the product is less than 5% by weight,and preferably less than 2.5% by weight. For example, when using thepreferred perfluorinated vinyl ether compound, it is present in thefluorinated ionomer in an amount by weight of about 0.3% to about 5.0%,and preferably less than about 2.5%.

The preparation of the mini-emulsion depends on careful selection of theingredients. The mini-emulsion is prepared by mixing water,perfluorinated hydrocarbon, fluorinated surfactant(s), ionomer,co-surfactant or inorganic salts, and the vinyl ether compound. Theamounts employed are 0.1-40 weight percent, preferably 0.1-20, of theperfluorinated hydrocarbon; 1-40 weight percent, preferably 0.1-25, ofthe surfactant and cosurfactants; 1-20 weight percent, preferably 5-15,of the ionomer; 0.3-5 weight percent, preferably less than 2.5 weightpercent, with the remainder water. This mixture is subjected to highshear mixing using methods known in the art such as mechanical shearand/or cavitation to break the oil phase into submicron size droplets.Multiple passes through such mixers may be required to obtain amini-emulsion. The resulting mini-emulsion is neither completelytransparent as observed with microemulsions, nor milky white as it is ina (macro)emulsion. Rather, it is substantially translucent, often with aslight hint of color, for example a blue tint. Without being bound byany particular theory, the resulting mini-emulsion of perfluorinatedhydrocarbons is believed to serve as mini-reactors for fluorinatedmonomers to enter and to be polymerized.

To initiate polymerization, the temperature of the mini-emulsion isadjusted to between 0 and 150 degrees C., preferably 40 to 100 degreesC. Initiators for polymerization include free-radical initiators, suchas persulfates, azo initiators, peroxides, or photo initiators, whichcan generate free radicals by ultraviolet or gamma rays. Amount ofinitiators present can range between 0.001 to 5 percent by weight basedon the final polymer content. The fluorinated monomers are introduced tothe reactor either in vapor phase or liquid phase into the aqueousliquid. Sufficient mixing between phases is important to encourage masstransfer.

As will be understood by one well practiced in the art, otherpolymerization procedures may also be employed. In particular, the useof the perfluorinated hydrocarbon of low molecular weight as part of thepolymerization mixture is not necessarily required, as long as the lowlevels of branch forming agent are still employed. In particular, thegeneral polymerization process described in U.S. Pat. No. 3,282,875,U.S. Pat. No. 4,358,545, and that in WO/00/52060 to Bekarian, et. al.may be employed as long as the low levels of branch forming agent areemployed appropriately during polymerization.

The product produced from the polymerization is an ion conductingpolymer with low equivalent weight and relatively low hydration. Theresultant ionomers are nonetheless soluble in organic solvents, whichallows them to be formed into thin films, either alone (see film 50 inthe exemplary embodiment shown in FIG. 4) or in composites with othersubstrates to form a composite membrane (see substrate 60 and ionomer 61forming composite membrane 62 in the exemplary embodiment shown in FIG.5). Such other substrates may comprise a support of porous material suchas expanded polytetrafluoroethylene (ePTFE). As used herein, “porous”means having a plurality of interconnected passages and pathways.Solutions of the ionomer may be impregnated into the porous support bymethods known in the art, for example as described in U.S. Pat. Nos.5,547,551 and 5,599,614 to Bahar et. al.

Such films are useful as separator membranes in membrane electrodeassemblies (MEAs). As shown in the exemplary embodiment shown in FIG. 6,electrodes 70, 71 are adhered or otherwise attached to either side of amembrane 72 to form MEA 73. MEA 73 is in turn useful in a fuel cell 83,as shown in the exemplary embodiment shown in FIG. 7. Gas diffusionmedia 80 and 81 may optionally be attached or otherwise adhered to theelectrodes, and current collectors (not shown) may optionally beconnected to positive terminal 84 and negative terminal 85. Duringoperation the fuel enters the cell and reacts at the anode to generateelectrons that are collected at negative terminal 85. The electrons flowthrough an external load (not shown) to the cathode terminal 84. Theelectrons are used at the cathode together with the oxidizing species.Depending on the type of fuel and the type of oxidizing species,products may be formed in the anode compartment, the cathode compartmentor both. If present, these products are swept out of the cell with anyexcess fuel and/or oxidizing species that may optionally have been usedin the inlet gases. In another alternative embodiment, the electrodes inthe MEA may also contain the instant invention as one component of amulti-component electrode system. The inventive polymer is also usefulin other electrolytic cells.

The following procedures were used to characterize the ionomers preparedaccording the above description.

Membrane Formation

For examples below where membranes were required, and prior to theequivalent weight determination described below, the following procedurewas followed. The acid form of the polymer was obtained from thesulfonyl fluoride form of the polymer using practices well known in theart. Here, it was generated by completely hydrolyzing the sulfonylfluoride form of the polymer in KOH and then completely reacidifying inHNO₃. Approximately 2 g of solid ionomer pieces in the acid formweighing ˜0.05 g each were placed in a uniform pile between two sheetsof Kapton® polyimide film (DuPont High Performance Materials,Circleville, Ohio). The sandwich of material was placed between thepreheated fully open 64 in² platens of a Marsh Instruments PHI pneumaticpress. The platen temperature was set such that the temperature readingbetween the top and bottom platens when in contact with each other was165° C. The bottom platen was then raised until the upper sheet ofKapton film made contact with the top platen. The ionomer sample wasthen allowed to sit for 15 minutes. The sandwich was then compressed bycycling the pressure 3-5 times between approximately 1 ton for 10seconds and approximately zero tons for ten seconds. The pressure wasthen increased to 5 tons, then slowly increased to 10 tons over a periodof 90 to 120 seconds. Finally, the pressure was then increased to 20tons and the sample was held under 20 tons at 165° C. for 120 seconds toproduce a uniform, clear film, typically approximately 5 mil thick.

Degree of Hydration

A sample ˜1 cm by ˜1.5 cm in size was cut from a membrane and placed ina vacuum oven at 120 degrees C. for ˜70 hours at ˜3 inches of Hg using anitrogen bleed to maintain the pressure. The sample was removed, andweighed when cool (˜1 minute or less) to obtain the dry weight. Thesample was then placed in deionized water for 2 hours at roomtemperature. As used herein, room temperature is 23 degrees C. plus orminus two degrees. The sample was removed from water, patted dry with apaper cloth, and immediately weighed to obtain the hydrated weight. Thedegree of hydration in percent is calculated as${{Degree}\quad{of}\quad{Hydration}\quad\left( {{in}\quad\%} \right)} = \frac{100*\left\lbrack {{{Hydrated}\quad{Weight}} - {{Dry}\quad{Weight}}} \right\rbrack}{{Dry}\quad{Weight}}$Four samples for each membrane were tested, and the reported hydrationis the arithmetic mean of the four measurements.Equivalent Weight

The method used here to determine equivalent weight takes a measuredweight of dried ionomer solid and calculates an acid equivalent weightbased on the first inflection point of the titration curve near pH 7.Specifically for each sample, approximately 5 g of solid ionomer piecesweighing no more than 0.05 g each were dried in oven for at least twohours at 80° C. under full vacuum (˜2 in. Hg). The dried pieces wereremoved from the oven and placed in a capped container in order tominimize moisture pickup. After allowing the dried sample to cool toroom temperature in the capped container, approximately 0.15 g wasquickly weighed into a 100 ml titration cup. The sample of known dryweight was then allowed to soak in the titration cup for 15 minutes in 5ml of deionized water and 5 ml of ethanol. To the soaked sample, 55 mlof 2.0N NaCl solution were then added. A back titration method using aTIM900 Titration Manager (Radiometer Analytical S.A., Lyon, France) wasthen started beginning with the addition of 5 ml of 0.05N NaOH solution.The entire blend was then stirred for 15 minutes under a nitrogenblanket prior to the acid titration with 0.01N HCl solution. The endpoint near pH 7 was used to calculate both the ion exchange capacity(IEC) and the acid equivalent weight (EW) of the sample according to${{IEC}\quad\left( {{meq}\text{/}g} \right)} = \frac{\left\lbrack {5\quad{ml}\quad{NaOH}\quad \times 0.05\quad N} \right\rbrack - \left\lbrack {{Volume}\quad{HCl}\quad({ml}) \times 0.01\quad N} \right\rbrack}{\left. {{Dried}\quad{Ionomer}\quad{Solid}\quad{Weight}\quad(g)} \right\}}$ EW(g/eq)=1000/IEC(meq/g)The arithmetic mean of the measured results from two different samplesof each membrane is reported as the Equivalent Weight.Room Temperature Ionic Conductivity

A membrane sample about 1.5 inches by about 2 inches in size was firstequilibrated at room conditions of 21 degrees C., 61% RH for 24 hrs. Itwas then immersed into a plastic beaker containing room temperaturedeionized water. Three measurements were taken over 90 minutes, oneevery 30 minutes. To take the measurements the membrane sample was takenout of the water and patted dry by paper tissues. The thickness was thenmeasured immediately using an MT12B Heidenhain (Schaunburg, Ill.)thickness gauge attached to a Heidenhain ND281B digital display. Thegauge was mounted vertically over a flat plate, and measurements weremade at nine different locations on the sample, covering the corners andcenter of the sample. The spring-loaded probe of the gauge was loweredgently on the film for each measurement to minimize compression. Themean of the nine values was used as the sample thickness. The ionicresistance of the membrane, 11, was then measured using a four-pointprobe conductivity cell shown in FIG. 1. The sensing probes, 5, ofconductivity cell, 10, are approximately one inch long, andapproximately one inch apart. A Plexiglas spacer 1 provides insulationbetween the current probes 4 and sensing probes 5. The cell is heldtogether with nylon screws 2 and electrical contact is made to theprobes through holes 3. During the measurement, a 500 g weight (notshown) was loaded onto the cell to ensure good contact. It was foundthat the resistance value is independent of further pressure ontoconductivity cell 10. The resistance was measured by connecting leads(not shown) through holes 3 using 10 mV AC amplitude at 1000 Hzfrequency applied by a Solartron SI 1280B controlled by ZPlot softwarewritten by Scribner Associates. Measurements were taken in thepotentiostatic mode. Under these conditions, the phase angle was foundto be insignificant throughout the measurement. The room temperatureionic conductivity in S/cm for each measurement was calculated from theformula $\sigma = \frac{L_{2}}{R*L_{1}*D}$

Where σ is the room temperature ionic conductivity, L2 is distancebetween the sensing probes, here equal to 2.5654 cm, L1 is the length ofthe sensing probe, here 2.5603 cm, D is the measured thickness of themembrane in cm, and R is the measured resistance in ohms. The resultsshowed that the room temperature ionic conductivity was independent ofthe soaking time between 30 and 90 minutes for all the samples tested.The reported value is the average calculated from the threemeasurements.

High Temperature Ionic Conductivity

The high temperature ionic conductivity at temperatures of 80 and 120degrees C. was also measured. In this case, the conductivity wasmeasured using a different apparatus where the temperature and relativehumidity of the atmosphere could be more precisely controlled. Thesemeasurements were performed to confirm that the conductivity of samplessoaked in room temperature water and measured at room temperature showedthe same trends between materials as samples measured at highertemperature and equilibrated at a fixed relative humidity condition.These measurements are particularly relevant because it is well knownthat Schröder's paradox [P. Schröder, Z. Physik Chem., Vol. 75, pg. 75(1903)] is observed in perfluorosulfonic acid type ionomeric membranes[e.g., see T. A. Zawodzinski, T. E. Springer, F. Uribe and S.Gottesfeld, Solid State Ionics, Vol. 60, pg. 199 (1993) and G.Blumenthal, M. Cappadonia and M. Lehmann, Ionics, Vol. 2, pg. 102(1996)]. It is thus expected that the measured conductivity of theinventive membrane will be different when measured in liquid watercompared to that measured in 100% relative humidity at the sametemperature even though the water activity is, in theory, equal to onein both cases. Therefore, to confirm that the inventive ionomers doindeed have improved conductivity when in equilibrium with water vaporas well as when soaked in liquid water, a high temperature conductivitytest was performed where the relative humidity and temperature werecontrolled.

This test was performed as follows: three different thicknesses of thesample membrane to be tested were prepared as described above. Two 0.5inch diameter ELAT® gas diffusion media (GDM) available from E-TEK, Inc.were die cut. Approximately 1 mg/cm² of ionomer prepared according toExample 2 in co-pending application to Wu et. al., was brushed onto theGDM surface, and then placed against an ˜1.5 inch by ˜1.5 inch samplemembrane to form a sandwich. This sandwich was then laminated for 3minutes by applying 15 tons of pressure to ˜18 inch by ˜18 inch platensof a hydraulic press where the top platen was heated to 160 degrees C.After cooling, the GDM/sample/GDM sandwich was placed in the hightemperature ionic conductivity apparatus 20 as shown in FIG. 2 fortesting.

The apparatus 20 consists of a split aluminum body 21 with apolytetrafluoroethylene (PTFE) cell 22. Cell 22 is clamped togetherduring operation with an air-actuated pressure of 120 psi. Two electrodeleads 23 with 40% porous 316L stainless steel Pt coated pellets 24welded on the end of the electrode leads 23 enter the cell 22 throughthe center to form the two electrodes between which the test sample 25is placed. Test sample 25 comprises sample membrane 26 with GDM 27 onboth sides prepared as described above. The bottom electrode lead 23 isattached to an air actuated cylinder (not shown) that can apply a fixedpressure to the bottom electrode lead. A pressure of 150 psi was usedfor all testing described herein. Nitrogen gas is flowed into the cellthrough two lines, one for each half of the cell. The humidity of theeach inlet gas stream is controlled by flowing the gas through a bottleof water where the temperature is fixed. The gas lines after the eachhumidification bottle are also heated to prevent condensation. The celltemperature, temperature of the humidification bottles, and gas linesare controlled by a Scribner Associates Membrane Test System (ScribnerAssociates, North Carolina). The humidity of both inlet gas streams ismeasured with a Vaisala HM138 humidity probe (Vaisala Group, Vantaa,Finland). For all testing here, the measured humidity of both halves ofthe cell was the same to within 3-5 percent RH.

After placing the test sample 25 in the cell, closing it, and applyingpressure to the cell and the electrode leads, the cell was heated to thelowest test temperature with flowing dry gas. It was equilibrated underdry gas at that temperature for 30 minutes. Then the humidity wasstepped to 10% RH. The frequency for measurements was then determined bymeasuring the impedance in potentiostatic mode with a frequency sweepfrom ˜1 Hz to ˜20 kHz using a Solartron 1280B Impedance Analyzer(Solartron Analytical, Hampshire, England). The frequency where themeasured phase angle was about zero was determined. This frequency wasused in all subsequent measurements. Typically, this frequency was inthe range of 7 to 15 kHz. The test sequence was then initiated undercomputer control whereby the humidity was changed at low temperatures tothe following values: about 10, 20, 40, 50, 60, 80, and 90 percent RH.Impedance measurements at each RH step were recorded at five secondintervals until the impedance changed less than 1 milliohm. This steadystate impedance (equal to the resistance since the phase angle is zero)was recorded as the cell resistance at that temperature and RH. Then thecell is stepped to a new higher temperature, returned to 10% RH, and theprocess repeated. For higher temperatures, it was not possible to reachhigh relative humidities because the cell was not operated underpressure. Therefore, the maximum RH achievable was lower at highertemperatures. In this case, six steps of RH were made between 10% andthe maximum achievable RH at that temperature. The RH and temperatureare reported here as the average of the two values obtained from the RHand thermocouple probes in each half of the cell.

In order to remove the effects of interfacial resistances, which can bea significant fraction of the total resistance, the resistance at anygiven temperature/RH condition was measured for samples of threedifferent thicknesses. These resistances were plotted as a function ofthickness, a linear regression fit to the data, and the extrapolatedzero thickness resistance value was used as the interfacial resistancefor that sample. This value was then subtracted from the measuredresistance to obtain the actual sample resistance. The high temperatureionic conductivity was calculated from the formula:$\sigma = \frac{L}{R*A}$

-   -   where L is the thickness of the sample measured before it is        placed in the apparatus, and A is the area of the sample in        contact with the electrode leads, i.e., π times the diameter        squared divided by 4; and R is the measured resistance reduced        by the interfacial resistance determined from the zero thickness        extrapolation.

The following examples are intended to demonstrate but not to limit theinventive compounds and methods of making them.

EXAMPLE 1

A divinyl ether compound was formed by reacting 2,2-difluoromalonic acidfluoride (O═CF—CF₂—CF═O) and hexafluoro propylene oxide. The2,2-difluoromalonic acid fluoride was prepared by direct fluorination ofmalonic acid by F₂ gas. The addition reaction of one molecule of2,2-difluoromalonic acid fluoride and two molecules of hexafluoropropylene oxide produced one molecule ofO═CF—CF(CF₃)—O—CF₂CF₂CF₂—O—CF(CF₃)CF═O,which after a standard decarboxylation reaction became the desiredbranch forming monomer, CF₂═CF—O—CF₂CF₂CF₂—O—CF═CF₂. The product wasreacted to a more stable form for long-term storage by flowing brominegas through the product to saturate the vinyl groups with bromine,forming CF₂BrCFBrOCF₂CF₂CF₂OCFBrCF₂Br. The vinyl form was regeneratedseveral days prior to use according to the following procedure: 120grams of zinc powder and 200 ml of tetraglyme were charged into a 1 Lflask equipped with a stirrer, thermometer, reflux condenser anddropping funnel. After heating to 100 degrees C., 200 ml of thebrominated version of branching agent was added dropwise over about 2hours by way of the dropping funnel to the stirring solution in theflask. The product was distilled under a reduced pressure of 150 ml Hgat 49 degrees C. Two hundred and five (205) grams of the vinyl etherbranching agent, CF₂═CF—O—CF₂CF₂CF₂—O—CF═CF₂, was obtained for a yieldof about 90%.

An aqueous mini-emulsion was prepared by pre-mixing and homogenizationof a mixture containing 1650 grams of deionized water, and 50 grams of20% by weight of ammonium perfluoro octanoate (ammonium salt ofperfluoro octanoic acid, manufactured by 3M) aqueous solution, and 194grams of 82.5% by weight of sulfonyl fluoride monomer having the formulaCF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂—SO₂F, and 17.5% by weight of Fluorinert®FC-77 (a perfluorinated hydrocarbon manufactured by 3M), and 1 gram ofCF₂═CF—O—CF₂CF₂CF₂—O—CF═CF₂, the vinyl ether compound described above. Amini-emulsion was formed using the homogenizing module of amicrofluidizer. An air motor using compressed air of about 40 psi sentthe mixture through the homogenizing module. The whole mixture was sentthrough the homogenizing module six times. The final mixture was atranslucent aqueous mini-emulsion of very light blue color. In a 4-literpressure reactor, the aqueous mini-emulsion was added to the reactor.Then, the reactor was evacuated three times and purged each time withtetrafluoroethylene gas. The oxygen content of the aqueous solution wasabout 20 ppm immediately prior to admitting the tetrafluoroethylene gas.

The reactor agitation speed was set at 700 rpm throughout the reaction.The aqueous mini-emulsion was heated from the jacket to a temperature ofabout 70 degrees C. Then, tetrafluoroethylene gas was introduced to thepressure reactor and the pressure was raised to about 0.5 MPa. About 0.1gram of ammonium persulfate pre-dissolved in 400 ml of deionized waterwas pumped into the reactor to start the reaction. The reactiontemperature was maintained between 69 and 71 degrees C. Thetetrafluoroethylene pressure was maintained relatively constant between0.48 and 0.55 MPa for the first 2 hours of polymerization reaction, withcontinuous charge of tetrafluoroethylene gas to the reactor tocompensate for the consumption of tetrafluoroethylene forcopolymerization. After the 2 hours of polymerization reaction, thetetrafluoroethylene supply was stopped, and the reaction continuedwithout more charge of tetrafluoroethylene to the reactor. The reactionpressure went down gradually from 0.48 MPa to 0.20 MPa in about 90minutes. Then, the reaction temperature was lowered to below 50 degreesC. and the reaction system was evacuated to atmosphere. The reactionyielded an aqueous dispersion of about 2.29 kg. The total polymerobtained by precipitation and isolation was about 5.4% by weight of thedispersion product. The final polymer was calculated to contain about0.8% by weight (0.47 mole %) of the divinyl ether compound, assumingcomplete incorporation of the divinyl monomer into the polymer. TheEquivalent Weight of this sample was about 690. The concentration ofpendant groups derived from the ionomeric monomer and the vinyl ethermonomer is about 20%.

EXAMPLE 2

An aqueous mini-emulsion was prepared by pre-mixing and homogenizationof a mixture containing 1650 grams of deionized water, and 50 grams of20% by weight of ammonium perfluoro octanoate (ammonium salt ofperfluoro octanoic acid) aqueous solution, and 186 grams of 85% byweight of CF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂—SO₂F monomer and 15% by weight ofFluorinert® FC-77 (a perfluorinated hydrocarbon manufactured by 3M), and1 gram of CF₂═CF—O—CF₂CF₂CF₂—O—CF═CF₂, prepared as described inExample 1. In a 4-liter pressure reactor, the aqueous mini-emulsion wasadded to the reactor. Then, the reactor was evacuated three times andpurged each time with tetrafluoroethylene gas. The oxygen content of theaqueous solution was about 20 ppm immediately prior to admitting thetetrafluoroethylene gas.

The reactor agitation speed was set at 700 rpm throughout the reaction.The aqueous mini-emulsion was heated from the jacket to a temperatureabout 60 degree C. Then, tetrafluoroethylene gas was introduced to thepressure reactor and the pressure was raised to about 0.5 MPa. About 0.1gram of ammonium persulfate pre-dissolved in 400 ml of deionized waterwas pumped into the reactor to start the reaction. After 30 minutes, thereaction temperature was increased and maintained between 65 and 66degrees C. The tetrafluoroethylene pressure was maintained at arelatively constant pressure between 0.52 and 0.56 MPa for the next 90minutes of reaction, with continuous charge of tetrafluoroethylene gasto the reactor to compensate the consumption of tetrafluoroethylene forcopolymerization. Then, the reaction temperature was increased again andmaintained between 69 and 71 degree C. The tetrafluoroethylene pressurewas maintained relatively constant between 0.49 and 0.57 MPa for thenext 150 minutes of reaction, with continuous charge oftetrafluoroethylene gas to the reactor to compensate for the consumptionof tetrafluoroethylene for copolymerization. Finally, thetetrafluoroethylene supply was stopped and the reaction continued at atemperature of about 70 degrees C. without more charge oftetrafluoroethylene to the reactor. The reaction pressure went downgradually from 0.48 MPa to 0.45 MPa in about 20 minutes. Then, thereaction temperature was lowered to below 50 degrees C. and the reactionsystem was evacuated to atmosphere. The reaction yielded an aqueousdispersion of about 2.32 kg. The total polymer obtained by precipitationand isolation was about 5.4% by weight of the dispersion product. Thefinal polymer was calculated to contain about 0.8% by weight (0.47 mole%) of the divinyl ether compound, assuming complete incorporation of thedivinyl monomer into the polymer. The Equivalent Weight of this samplewas about 690. The concentration of pendant groups derived from theionomeric monomer and the vinyl ether monomer is about 20%.

EXAMPLE 3

An aqueous mini-emulsion was prepared by pre-mixing and homogenizationof a mixture containing 1650 grams of deionized water, and 50 grams of20% by weight of ammonium perfluoro octanoate (ammonium salt ofperfluoro octanoic acid) aqueous solution, and 160 grams ofCF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂—SO₂F monomer and 40 grams of Fluorinert®FC-77 (a perfluorinated hydrocarbon manufactured by 3M), and 1 gram ofCF₂═CF—O—CF₂CF₂CF₂—O—CF═CF₂, prepared as described in Example 1. In a4-liter pressure reactor, the aqueous mini-emulsion was added to thereactor. Then, the reactor was evacuated three times and purged eachtime with tetrafluoroethylene gas. The oxygen content of the aqueoussolution was about 20 ppm right immediately prior to admitting thetetrafluoroethylene gas.

The reactor agitation speed was set at 700 rpm throughout the reaction.The aqueous mini-emulsion was heated from the jacket to a temperatureabout 70 degrees C. Then, tetrafluoroethylene gas was introduced to thepressure reactor and the pressure was raised to about 0.5 MPa. About 0.1gram of ammonium persulfate pre-dissolved in 400 ml of deionized waterwas pumped into the reactor to start the reaction. The reactiontemperature was maintained between 69 and 71 degrees C. Thetetrafluoroethylene pressure was maintained relatively constant between0.48 and 0.51 MPa for the first 3 hours of reaction, with continuouscharge of tetrafluoroethylene gas to the reactor to compensate for theconsumption of tetrafluoroethylene for copolymerization. After the 3hours of polymerization reaction, the tetrafluoroethylene supply wasstopped and the reaction continued without more charge oftetrafluoroethylene to the reactor. The reaction pressure went downgradually from 0.48 MPa to 0.20 MPa in about 3 hours. Then, the reactiontemperature was lowered to below 50 degrees C. and the reaction systemwas evacuated to atmosphere. The reaction yielded an aqueous dispersionof about 2.31 kg. The total polymer obtained by precipitation andisolation was about 8.0% by weight of the dispersion product. The finalpolymer was calculated to contain about 0.5% by weight (0.30 mole %) ofthe divinyl ether compound, assuming complete incorporation of thedivinyl monomer into the polymer. The Equivalent Weight of this samplewas about 690. The concentration of pendant groups derived from theionomeric monomer and the vinyl ether monomer is about 20%.

EXAMPLE 4

An aqueous mini-emulsion was prepared by pre-mixing and homogenizationof a mixture containing 1650 grams of deionized water, and 50 grams of20% by weight of ammonium perfluoro octanoate (ammonium salt ofperfluoro octanoic acid) aqueous solution, and 160 grams ofCF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂—SO₂F monomer and 40 grams of Fluorinert®FC-77 (a perfluorinated hydrocarbon, manufactured by 3M), and 2.5 gramsof CF₂═CF—O—CF₂CF₂CF₂—O—CF═CF₂, prepared as described in Example 1. In a4-liter pressure reactor, the aqueous mini-emulsion was added to thereactor. Then, the reactor was evacuated three times and purged eachtime with tetrafluoroethylene gas. The oxygen content of the aqueoussolution was about 13 ppm immediately prior to admitting thetetrafluoroethylene gas.

The reactor agitation speed was set at 700 rpm throughout the reaction.The aqueous mini-emulsion was heated from its jacket to a temperatureabout 70 degrees C. Then, tetrafluoroethylene gas was introduced to thepressure reactor and the pressure was raised to about 0.5 MPa. About 0.1gram of ammonium persulfate pre-dissolved in 400 ml of deionized waterwas pumped into the reactor to start the reaction. The reactiontemperature was maintained between 69 and 71 degrees C. Thetetrafluoroethylene pressure was maintained at a relatively constantpressure between 0.45 and 0.51 MPa for the first 3 hours of reaction,with continuous charge of tetrafluoroethylene gas to the reactor tocompensate for the consumption of tetrafluoroethylene forcopolymerization. After the 3 hours of polymerization reaction, thetetrafluoroethylene supply was stopped, and the reaction continuedwithout more charge of tetrafluoroethylene to the reactor. The reactionpressure went down gradually from 0.47 MPa to 0.31 MPa in about 1 hour.Then, the reaction temperature was lowered to below 50 degrees C. andthe reaction system was evacuated to atmosphere. The reaction yielded anaqueous dispersion of about 2.30 kg. The total polymer obtained byprecipitation and isolation was about 5.0% by weight of the dispersionproduct. The final polymer was calculated to contain about 2.2% byweight (1.29 mole %) of the divinyl ether compound, assuming completeincorporation of the divinyl monomer into the polymer. The EquivalentWeight of this sample was about 710. The concentration of pendant groupsderived from the ionomeric monomer and the vinyl ether monomer is about19%.

EXAMPLE 5

A membrane was formed from the fluorinated ionomeric co-polymer productof Example 2 using the film formation procedure described above. Theequivalent weight, degree of hydration and room temperature conductivityof this film was measured according to the procedures described above.The results are presented in Table 1.

EXAMPLE 6

A membrane was formed from the fluorinated ionomeric co-polymer productof Example 2 using the film formation procedure described above exceptthat the films were pressed using a temperature of 120 degrees C.instead of 165 degrees C. The equivalent weight, degree of hydration andionic conductivity of this film was measured according to the proceduresdescribed above. The results are presented in Table 1. Comparing theresults of Example 5 and 6 show that the film formation method does notsubstantially effect the properties of the resultant membrane.

COMPARATIVE EXAMPLE A

A Nafion® 112 membrane was purchased from Du Pont Co. It was tested asreceived to determine the equivalent weight and hydration. Theconductivity was tested as described above except two measurements at 30and 60 minutes were made in the machine direction on one piece of film,and a third measurement at 90 minutes was made in the transversedirection of a second piece of film. No significant different wasobserved between the conductivity of the two directions as expected fromprior literature [see, for example, G. Blumenthal, M. Cappadonia, M.Lehman, “Investigation of the Proton Transport in Nafion® Membranes as aFunction of Direction, Temperature and Relative Humidity”, Ionics,Volume 2, pg. 102-106 (1996)]. The room temperature ionic conductivitywas measured as described above and the reported room temperature ionicconductivity is the average of these three measurements (Table 1). Thehigh temperature ionic conductivity was measured as described aboveexcept the ionomer that was brushed on the GDM was Nafion 1100 insteadof that described above. The conductivity and hydration results areconsistent with those widely reported in the literature for thiscommercial material [see, for example, T. Zawodinski, C. Derouin, S.Radzinski, R. Sherman, V. Smith, T. Springer and S. Gottesfeld, Journalof the Electrochemical Society, Volume 140, No. 4, 1041-1047 (1993)],confirming that the measurement technique is satisfactory.

COMPARATIVE EXAMPLE B-F

The data from Table 1 in WO 00/52060 for equivalent weights 1100, 980,834 and 785 is reported directly as Comparative Example B, C, D, and E,respectively. Comparative Example F in Table 1 reports the data directlyfrom inventive Example 9 of WO 00/52060. The conductivity and wateruptake reported in WO 00/52060 were obtained using essentially the sameprocedure as that used here, so the data is directly comparable to theExample 5 and 6. The conductivity is substantially higher, and thedegree of hydration substantially lower for the instant inventions ofthis application relative to the prior art of these ComparativeExamples.

COMPARATIVE EXAMPLE G

A sample was prepared according to the procedure given in Example 1 ofco-pending application to Wu, et. al., entitled Low Equivalent WeightIonomers. A membrane was formed from this polymer using the proceduredescribed above. This membrane was very fragile compared to those inExample 5 and 6. The equivalent weight, degree of hydration and roomtemperature conductivity of this membrane were measured (Table 1). Theequivalent weight of this Comparative Example is approximately the sameas the instant invention illustrated in Examples 5 and 6. These resultsshow that for a given equivalent weight, the instant invention of thisapplication has improved physical stability when hydrated, asubstantially higher conductivity and a substantially lower degree ofhydration compared to the prior art illustrated in this ComparativeExample. TABLE 1 Equivalent Weight, Conductivity and Hydration ofInventive and Comparative Examples Equivalent Degree of Mean WeightHydration Conductivity (eq./g) (%) (S/cm) Example 5 689 74.8 0.234Example 6 691 74.8 0.212 Comparative 1104 16.82 0.083 Example AComparative 1100 25 0.0902 Example B Comparative 980 27.1 0.1193 ExampleC Comparative 834 53.1 0.1152 Example D Comparative 785 79.2 0.0791Example E Comparative 1156 15 0.065 Example F Comparative 698 118.50.145* Example G*Average of only two measurements. Sample tore after second measurement,making a third measurement impossible.

COMPARATIVE EXAMPLE H

A sample was prepared according to the procedure described in Example 2of co-pending application to Wu et. al., entitled Low Equivalent WeightIonomers. A membrane of this polymer was prepared as described above.This polymer was found to have an equivalent weight of 810 and a degreeof hydration of 42.3%. The high temperature conductivity of this samplewas measured, and compared to that of Example 6, and Comparative ExampleA. Results show that the conductivity of the inventive polymer disclosedhere is higher under all measured conditions than both of theComparative Examples. TABLE 2 High Temperature Ionic ConductivityResults at Various Temperatures and Relative Humidities Mean IonicConductivity (S/cm) 80° C. 80° C. 120° C. 120° C. 20% RH 80% RH 10% RH40% RH Comparative 0.003 0.044 0.001 0.0019 Example A Comparative 0.0060.057 0.003 0.036 Example H Example 6 0.014 0.081 0.011 0.066

COMPARATIVE EXAMPLE I

A sample was prepared according to the procedure described in Example 5of co-pending application to Wu et. al., entitled Low Equivalent WeightIonomers. This polymer was found to have an equivalent weight of 838 anda degree of hydration of 36.7%.

EXAMPLE 7 AND COMPARATIVE EXAMPLE J AND K

Membrane electrode assemblies (MEAs) were made and tested to demonstratethe utility of instant invention for use in fuel cells. Twocorresponding Comparative Examples were also prepared. The first,Comparative Example J, used a commercially available MEA similar to theones prepared here. The second MEA, Comparative Example K, was preparedusing the ionomer of Comparative Example I. Comparative Example K andExample 7 were prepared in the same way, except using differentionomers, the former from Comparative Example H, the latter from Example2. The MEAs of these two samples were prepared for testing as follows:the ionomer in its hydrolyzed and acidified form was first solubilizedin ethanol to form a solution containing 10% ionomer. This solution wasthen impregnated into a 22.5 micron thick support of ePTFE according tothe teachings of Bahar, et. al. in U.S. Pat. No. RE37,307. The ePTFE wasfixed in a 10-in embroidery hoop. The ionomer solution was painted onboth sides of the ePTFE and then dried with a hair drier to remove thesolvent. The painting and drying steps were repeated 2 more times. TheePTFE and the embroidery hoop were then placed into a solvent oven at180° C. for 8 minutes. The sample was then removed and allowed to coolto room temperature. One more coat of ionomer solution was painted onboth sides. The sample was placed back into the oven at 180° C. for 8minutes. The sample was then removed from the oven and taken off of theembroidery hoop. The ePTFE/ionomer composite membrane was transparent,indicating substantially complete impregnation of the support by theionomer.

An electrode containing 0.4 mg Pt/cm² and available from W. L. Gore &Associates, Inc. as part of its MEA bearing the designation Primea® 5510(available from Japan Gore-Tex Inc., Japan) was laminated to both sidesof the composite membrane. The electrode was first laid down over an0008 inch thick ePTFE bottom sheet. The composite membrane was then laiddown over the electrode, and another electrode was laid down over themembrane. Then a 0.005 inch thick ePTFE top sheet was laid down over theelectrode. The assembly was pressed at 160° C. at 15 tons of pressurefor 3 minutes, then the top and bottom ePTFE sheets were peeled off anddiscarded.

Comparative Example J used a PRIMEA® membrane electrode assembly Series5510, commercially available W. L. Gore and Associates. This assemblyused the same electrodes as in Example 7 and Comparative Example K and asimilar ePTFE reinforcement in the electrolyte. The only substantivedifference between Example 7 and Comparative Example I and J, then, wasthe ionomer in the electrolyte.

Cells using the three MEAs were assembled anode side first. Asilicone-coated fiberglass gasket 0.007 inches thick with an innerwindow of 52.5 cm² was first placed down on top of a quadrupleserpentine graphite anode flow field available from Fuel CellTechnologies (50 cm², 8 bolt fuel cell test hardware available from FuelCell Technologies was used). On top of the silicone-coated fiberglassgasket was placed a 0.0012 inch thick OL-12 spacer (Mylar film availablefrom DuPont) with an inner window of 52.5 cm² aligned with the innerwindow of the silicone-coated fiberglass gasket. Next, a single-sidedElat gas diffusion media (GDM) available from E-Tek, having a 52 cm²area and being about 0.014 to 0.015 inches thick, was placed inside theinner windows of the silicone-coated fiberglass gasket and the spacerwith the carbon side facing up. Next, a 0.0012 inch thick OL-12sub-gasket having an inner window of 45 cm² was placed on top of theGDM, followed by the MEA. This gasket reduced the active area of thecell to 0.45 cm². The above steps were repeated in the opposite order ontop of the MEA. Once an MEA “sandwich” was created, a cathode flow field(same as the anode flow field described above) was placed on top. Thebolts were lubricated with Krytox grease (available from DuPont) andtightened in a star pattern in 5 in-lb bolt increments until each boltachieved 75 in-lb of torque. The components used yielded an active areacompression of 150 to 200 psi.

The test station used was a Globe Tech gas unit with a Scribner 890load. Three-liter humidification bottles were used on the anode andcathode, and all lines coming into the cell were heat traced (heatedalong their length). Once the cell was hooked up to the test station,the fuel gasses were applied (H₂ on anode at 1.3 stoichiometry and airon cathode at 2.0 stoichiometry). The cell was then set to 60° C. andboth the anode and cathode bottles were set to 60° C. as well. The backpressure was kept at 0 psig on both sides. Once the temperatures came upto their respective set points, an automatic cycling program was run to“break in” the cell. The conditions for this cycling program are setforth in Table 3. TABLE 3 Start-up Procedure Time Anode Cathode Spent atHumidifi- Humidifi- Each cation cation Voltage Condition Temp. Temp.Anode Cathode (V) (min) (C.) (C.) RH* RH** 0.60 180 60 60 65%  85% 0.5030 60 60 65%  85% 0.95 0.5 60 60 65%  85% 0.70 30 60 60 65%  85% 0.950.5 60 60 65%  85% 0.50 30 65 65 80% 100% 0.95 0.5 65 65 80% 100% 0.7030 65 65 80% 100% 0.95 0.5 65 65 80% 100% 0.50 30 65 65 80% 100% 0.950.5 65 65 80% 100% 0.70 30 65 65 80% 100% 0.95 0.5 65 65 80% 100% 0.5030 65 65 80% 100% 0.95 0.5 65 65 80% 100% 0.70 30 65 65 80% 100% 0.950.5 65 65 80% 100% 0.50 30 65 65 80% 100% 0.95 0.5 65 65 80% 100% 0.6 3060 60 65%  85%*Relative humidity assuming the anode humidifier is 65% efficient.**Relative humidity assuming the cathode humidifier is 85% efficient.

Subsequent to finishing the cycling in Table 3, the cell was set to 80°C. cell temperature, 83° C. anode humidification, 51° C. cathodehumidification, and 7 psig back pressure on both the anode and cathode.This yielded an anode inlet relative humidity (RH) of 75% and a cathodeinlet RH of 25%, assuming the anode humidifiers are 65% efficient andthe cathode humidifiers are 85% efficient. Once the temperatures andpressures reached their respective set points, an automatic “sensitivityprotocol” was begun to test the MEA at various humidities. The“sensitivity protocol” is a program designed to determine how an MEAwill respond to changing humidity conditions. It is particularlydesigned to show the effects of cell operation in relatively dryconditions. TABLE 4 Fuel Cell Humidity Sensitivity Protocol and VoltageResults Com- Com- para- para- tive tive Anode Cathode Ex- Ex- Ex- Humid-Humid- ample ample ample ity ity Cath- J K 7 Temp. Anode Temp. ode MeanMean Mean Step (C.) RH* (C.) RH** Voltage Voltage Voltage 1 83 75% 5125% 0.685 0.686 0.705 2 86 100%  65 50% 0.684 0.682 0.704 3 86 100%  —†dry 0.658 0.682 0.696 4 83 75% —† dry 0.596 0.606 0.667 5 83 50% —† dry0.552 0.570 0.643*Relative humidity assuming the anode humidifier is 65% efficient.**Relative humidity assuming the cathode humidifier is 85% efficient.†Gas not humidified.

The protocol shown in Table 4 was followed. For each set of humidityconditions in Table 4, the cell was operated for two hours at a constantcurrent density of 800 mA/cm². The voltage during this time wasrecorded, and the mean of this two-hour voltage-time data was calculatedand recorded. Following the two-hour constant current hold, apolarization curve was recorded (not reported here). The polarizationcurve was obtained by measuring the steady state voltage after 10 or 20minutes (longer time for dry cathode conditions) following sequentialsteps to each of the following current densities: 0.8, 1.0, 1.2, 1.4A/cm². Then the current density was stepped to 0.8 A/cm² and the opencircuit voltage (i.e., no load applied to the cell) was measured after1.5 minutes. Then the rest of the polarization curve was obtained bymeasuring the steady state voltage after 10 or 20 minutes (longer timefor dry cathode conditions) following sequential steps to 0.6, 0.4, 0.2A/cm². Finally, the steady state voltage was measured after 5 or 13minutes (longer time for dry cathode condition) at 0.1 A/cm². Thetemperatures of the anode and cathode humidity bottles were then changedto the next condition shown in Table 4 to achieve the next RHconditions. An 800 mA/cm² constant current was applied, the voltage-timedata recorded, a mean voltage calculated, and a polarization curve takenas before. This procedure was repeated for each step in protocol shownin Table 4.

The average voltages observed for cells made from the inventive ionomerswhen tested under all 5 humidity conditions in Table 4 are significantlygreater than the voltages previously obtainable using ionomers such asthose used in Comparative Example J and K. The dramatic improvementobtained using the present invention under the conditions below 100% RHdemonstrates the utility of the ionomer when it comprises part of anMEA.

EXAMPLE 8-9 AND COMPARATIVE EXAMPLE L

In order to demonstrate the significant differences between theinventive ionomeric polymer, a dynamic mechanical analysis (DMA) studywas undertaken. Films were prepared as described above, except thestarting polymer was in the sulfonyl fluoride form instead of the acidform. Thin films formed from the polymer product described in Example 1,2 and Comparative Example G were prepared. These films were tested usinga standard strain controlled rheometer as described below, with theresults herein described as Examples 8, 9 and Comparative Example L,respectively.

The dynamic mechanical response was tested on a Rheometrics ScientificAres LS-M Rheometer (Piscataway, N.J.) using the standardtime-temperature superposition approach, as described in many standardtexts on polymer viscoelasticity, for example, in J. D Ferry,Viscoelastic Properties of Polymers, 3^(rd) Edition, J. Wiley & Son,1980. Specifically, 25 mm diameter solid thin films of ˜1-2 mm thicknesswere tested in a parallel plate geometry in a nitrogen atmosphere.Frequency sweeps between 0.1 and 100 rad/s were taken at 20 degree C.intervals at 5% strain for temperatures below 70 degrees C., and 10%strain for temperatures above 90 degrees C. The minimum and maximumtemperatures were adjusted from sample to sample so that the data waswithin the range of the instrument transducer. For the three samplestested in these examples, the minimum temperatures were 30, 30, and 10degrees C. for Examples 8, 9 and Comparative Example L, respectively;while the maximum temperatures were 150, 150, and 90 degrees C. The datafrom each temperature was reduced to a single master curve referenced at30 degrees C. using the Rheometrics Scientific Orchestrator software,version 6.5.6. The results, plotted in FIG. 3 in terms of the complexviscosity as a function of shear rate, show that the inventive polymersdescribed herein have a significantly higher viscosity at low shearrates. Without being bound by any particular theory, these results areconsistent with (though not conclusive proof of) the presence of longchain branching in the inventive materials.

1. A membrane electrode assembly comprising an anode and a cathodesandwiching a membrane comprising a fluorinated ionomer comprising atleast (a) a substantially fluorinated backbone; (b) pendant groupsderived from an ionomeric monomer of the formula

 where X is F, Cl or Br or mixtures thereof; n is an integer equal tozero, one or two; R_(f) and R_(f′) are independently selected from thegroup of F, Cl, perfluoroalkyl radical, and chloroperfluoroalkylradical; Y is an acid group or a functional group convertible to an acidgroup; a is zero or an integer greater than zero; and b is an integergreater than zero; and (c) pendant groups derived from a vinyl ethermonomer that has at least two vinyl ether groups of the form, CA₂═CB—O—,where the vinyl groups are separated by greater than four atoms; A isindependently selected from the group containing F, Cl, and H; and B isindependently selected from F, Cl, H and OR_(i), where R_(i) is abranched or straight chain alkane that may be partially, substantiallyor completely fluorinated or chlorinated; wherein the concentration ofthe pendant groups derived from the ionomeric monomer and the vinylether monomer is between about 10 mole percent and about 45 mole percentof the final product, and wherein the concentration of the pendantgroups derived from the vinyl ether monomer is present in the finalproduct between about 0.05 and about 2 mole percent.
 2. A membraneelectrode assembly as defined in claim 1 wherein said membrane furthercomprises a porous support and the fluorinated ionomer is impregnatedwithin said porous support.
 3. A membrane electrode assembly as definedin claim 2 wherein said porous support is expandedpolytetrafluoroethylene.
 4. A membrane electrode assembly as defined inclaim 2 wherein said fluorinated ionomer substantially completelyimpregnates said porous support.
 5. An electrochemical cell with acathode feed comprising an oxidizing species, an anode feed comprising areducing species relative to that in the cathode feed, saidelectrochemical cell comprising a membrane electrode assembly as definedin claim
 1. 6. An electrochemical cell with a cathode feed comprising anoxidizing species, an anode feed comprising a reducing species relativeto that in the cathode feed, said electrochemical cell comprising amembrane electrode assembly as defined in claim
 2. 7. An electrochemicalcell with a cathode feed comprising an oxidizing species, an anode feedcomprising a reducing species relative to that in the cathode feed, saidelectrochemical cell comprising a membrane electrode assembly as definedin claim
 3. 8. An electrochemical cell with a cathode feed comprising anoxidizing species, an anode feed comprising a reducing species relativeto that in the cathode feed, said electrochemical cell comprising amembrane electrode assembly as defined in claim
 4. 9. An electrolyticcell comprising a membrane as a membrane comprising a fluorinatedionomer comprising at least (a) a substantially fluorinated backbone;(b) pendant groups derived from an ionomeric monomer of the formula

 where X is F, Cl or Br or mixtures thereof; n is an integer equal tozero, one or two; R_(f) and R_(f′) are independently selected from thegroup of F, Cl, perfluoroalkyl radical, and chloroperfluoroalkylradical; Y is an acid group or a chloroperfluoroalkyl radical; Y is anacid group or a functional group convertible to an acid group; a is zeroor an integer greater than zero; and b is an integer greater than zero;and (f) pendant groups derived from a vinyl ether monomer that has atleast two vinyl ether groups of the form, CA₂═CB—O—, where the vinylgroups are separated by greater than four atoms; A is independentlyselected from the group containing F, Cl, and H; and B is independentlyselected from F, Cl, H and OR_(i), where R_(i) is a branched or straightchain alkane that may be partially, substantially or completelyfluorinated or chlorinated; wherein the concentration of the pendantgroups derived from the ionomeric monomer and the vinyl ether monomer isbetween about 10 mole percent and about 45 mole percent of the finalproduct, and wherein the concentration of the pendant groups derivedfrom the vinyl ether monomer is present in the final product betweenabout 0.05 and about 2 mole percent.
 10. A membrane electrode assemblycomprising an anode, a cathode and a membrane, wherein at least one ofsaid anode and cathode comprises the fluorinated ionomer of claim 1.